Great and fun topic! My ex and I found ourselves at a friend's party on night. We were engaged at the time, not yet married, and we were having regular sex. We hadn't yet discussed any kind of extra-marital topic except that she'd told me she'd had a crush on a friend of mine and thought they would have "made beautiful babies." I didn't know what to think of that, and just agreed with her. He's a good looking guy and smart so I guess she chose well as far as dreaming about who to have babies with.
There was quite a bit of drinking at the party and I hadn't even noticed that she'd disappeared for quite a while. After the party ended about 1 am, she and I went a few miles to a motel room I'd rented, not wanting to drive far after all that booze. I was horny as hell and she was pretty wasted. She had a dress on and climbed into bed that way. I was naked. She was resisting a bit as I began making out with her and trying to slip her panties down, but it was obvious I wasn't going to relent and she said "baby you probably don't want to do that." I asked why not and she said "because I'm dirty." It was then that I realized what had happened upstairs at the party, and I was hornier than I'd ever been. I didn't know why (at that age, I wasn't exactly a swinging man of the world) but I was excited beyond anything I'd ever experienced. I had my first "seconds" experience with the girl who would become my wife a few months later. So I guess you might say she "convinced me," as after we married, I encouraged her to have my friend again. We talked about it many times (about keeping the baby if she got pregnant with him) and it happened again in the extra bedroom at our home while I slept alone in our marital bed. By agreement.